1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a method for synthesizing metal cyanometallates (MCMs) for use in battery electrodes.
2. Description of the Related Art
The rechargeable lithium battery has triggered a portable electronic devices revolution due to their high power density, long cycling life, and environmental compatibility. The rechargeable lithium battery consists of a cathode (positive electrode) and an anode (negative electrode), separated by a lithium-ion (Li+) permeable membrane. A solution or polymer containing lithium-ions is also used in the battery to permit the lithium-ions to “rock” back and forth between the positive and negative electrode freely. The positive materials are mainly transition-metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and their derivatives. Lithium-ions can move in their interstitial space freely and reversibly. The negative electrode materials can be made from lithium-metal, alloys, and carbonaceous materials. When the rechargeable lithium battery does not include a metallic electrode, it is called as the lithium-ion battery. In the discharge process of the lithium-ion battery, Li+-ions are extracted from the negative electrode and inserted into the positive electrode. Meanwhile, electrons pass through an external circuit from the negative electrode to the positive electrode and generate electric power. In the charge process, ions and electrons move in the reverse direction and return to their original places.
Although lithium-ion batteries are widely used, lithium demand and its limited reserve surge its cost, which renders problematic the continuing application of lithium-ion batteries on a large scale. Therefore, a low-cost rechargeable battery alternative is needed. Under these circumstances, sodium-ion batteries are being investigated, because sodium has very similar properties to lithium, but a cheaper cost. Like lithium-ion batteries, sodium-ion (Na+) batteries need Na+-host materials as their electrode. Much effort has been expended to directly duplicate Li+-host structures as the Na+-host electrode materials for the sodium-ion batteries. For example, NaCoO2, NaMnO2, NaCrO2, and Na0.85Li0.17Ni0.21Mn0.64O2, having the similar layered-structure as LiCoO2, were developed for sodium-ion batteries. Similarly, Co3O4 with a Spinel structure, Na3V2(PO4)3 with a NASICON (Na3Zr2PSi2O12) structure, and NaFePO4 with an Olivine structure were employed in sodium batteries. In addition, sodium fluorophosphates, such as Na2PO4F, NaVPO4F and Na1.5VOPO4F0.5, were also used as the positive electrode in sodium batteries.
However, it is impractical to copy the structures of Li+-host compounds to Na+ or potassium-ion (K+)-host compounds. Sodium and potassium ions are much larger than lithium ions, and severely distort the structures of the Li+-host compounds. Thus, it is very important to develop new Na+/K+-host materials with large a interstitial space in which sodium/potassium-ions can easily and reversibly move. Na+/K+-ions have been observed to intercalate into metal cyanide compounds. Transition metal hexacyanoferrates (TMHCFs) with large interstitial spaces have been investigated as cathode materials for rechargeable lithium-ion batteries [1,2], sodium-ion batteries [3,4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates [(Cu,Ni)-HCFs] demonstrated a robust cycling life with 83% capacity retention after 40,000 cycles at a charge/discharge current of 17C (1C=150 milliamp hours per gram (mAh/g)) [6-8]. In spite of this, the materials demonstrated low capacities and energy densities because (1) only one sodium-ion can be inserted/extracted into/from per Cu-HCF or Ni-HCF formula, and (2) these TMHCF electrodes must be operated below 1.23 V due to the water electrochemical window. The electrochemical window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is important for the efficiency of an electrode, and once out of this range, water becomes electrolyzed, spoiling the electrical energy intended for another electrochemical reaction. To compensate for these shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were used as cathode materials in a non-aqueous electrolyte [9, 10]. When assembled with a sodium-metal anode, Mn-HCF and Fe-HCF electrodes cycled between 2.0V and 4.2 V delivered capacities ˜110 mAh/g.
FIG. 1 is a schematic diagram of the framework of ideal AxM1M2(CN)z (prior art). In general, the metal cyanometallates (MCMs) have the general formula of AxM1mM2n(CN)z that results in an open framework as shown in the figure. The open framework structure of the transition metal MCM facilitates both rapid and reversible intercalation processes for alkali and alkaline ions (Ax). The MCM capacity is determined by the available A-sites in the compounds into which the alkali and alkaline ions can be intercalated reversibly within the range of working voltages. From the electric neutrality point of view, the valences of M1 and M2 mainly contribute to the amount of the available A-sites. For example, 2 sodium-ions can be intercalated/deintercalated into/from Na2MnFe(CN)6 between 2-4V vs. Nao, because the valences of Mn- and Fe-ions can changed between +2 and +3, and its theoretical capacity is 171 mAh/g. However, for Na2FeCu(CN)6, only one sodium-ion per formula can be reversibly inserted/removed into/from the compound because the valence of Cu-ion cannot change between 2-4V vs. Nao. Its theoretical capacity is 83 mAh/g. In addition, it is inevitable that water and M1/M2-ions remain in the A-sites during synthesis because of the large interstitial spaces of the MCM compounds. For the purpose of increasing the sodium-ion concentration in Na2MnFe(CN)6, Dr. Goodenough's group used a high concentration of NaCl in the reaction solution in order to compete with water in occupying the interstitial positions and increase the capacity of the produced Na2MnFe(CN)6. Even so, only 118 mAh/g at a current of 12 mA/g was achieved [4]. Very recently, Berlin Green with a framework of FeFe(CN)6 was reported as the cathode material in sodium-ion batteries [11]. The material delivered a capacity of ˜115 mAh/g at a current of 120 mA/g. Noteworthy is the fact that the value was much smaller than its theoretical capacity of 200 mAh/g. The low capacity of Berlin Green can be attributed to the small Fe3+-ions interacting with its interstitial spaces. The interstitial Fe3+-ions definitely reduce the available positions for sodium-ion intercalation and, thus, the capacity of Berlin Green.
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